A conventional raster scanned cathode ray tube (CRT) display such as a television receiver or a computer visual display unit comprises circuitry that can generate electric fields of sufficient strength to radiate beyond the display. Various studies have raised public concern about these electric fields and the possible health hazards associated with them. As a result of these concerns various standards have been introduced defining maximum emission levels which products claiming to meet these standards can emit. In Northern Europe, for example, products can be tested to a standard developed and administered by TCO, the Swedish Confederation of Professional Employees. To meet a part of this standard, true RMS values of emissions in the frequency band from 2 kHz to 400 kHz are measured and must be less than 1 Volt/meter.
A CRT display typically comprises horizontal and vertical electromagnetic deflection coils arranged on a yoke mounted around the neck of the CRT. In operation, currents having a sawtooth waveform flow through the coils to scan the electron beam or beams across the CRT screen in a raster pattern. The voltages across the deflection coils reach a peak during the retrace or flyback period of the sawtooth currents. The peak voltage signals have a large component of harmonics of the corresponding deflection frequencies.
The electron beam or beams are accelerated from the neck of the CRT towards the screen by a "final anode" or Extra High Tension (EHT) voltage of typically 25 kV for a colour display. The flow of electrons is referred to as "beam current". The EHT voltage is typically generated from a step up transformer synchronised to the line scan. In displays having integrated horizontal deflection circuits and EHT generation, the voltage pulse signal driving the primary of the transformer is derived from the peak voltage across the horizontal deflection coil. In displays having separate EHT generation and horizontal deflection circuits, the voltage pulse signal is generated separately from the line scan signal, but may be synchronised to it, although not necessarily in phase.
The output impedance of the EHT generator is sufficiently high that changes in beam current loading through screen content cause modulation of the EHT voltage. This is the primary source of radiated electric fields in front of the display. This modulation of the internal CRT final anode voltage is coupled through the CRT faceplate and transmitted through the intervening medium (air in this case) to the observation point.
Electric field emissions from CRT displays can be reduced at the sides and back by enclosing the radiating conductors with grounded metal screens, and this is normal for multi-frequency displays. The screening necessary to reduce the emissions in front of the display is usually in the form of a conductive optical panel which is transparent to the light emitted by the CRT panel. The screen image is viewed through the panel which can diminish image quality. In addition, these panels are expensive to manufacture.
U.S. Pat. No. 5,151,635 describes an apparatus and method of reducing these time varying electric fields by providing a cancellation field of equal magnitude but opposite polarity to those generated by the horizontal deflection circuit, degaussing circuit and other circuits along with radiating antennae for each of these cancellation fields.
European Patent Application 0 523 741 describes a similar apparatus which senses the electric field associated with the deflection yoke and provides a signal to a radiating antennae.
For displays having integrated EHT generation and horizontal deflection circuits, the electric field sensed from the deflection circuit is similar to the actual electric field emitted from the display and so some cancellation of the primary source of radiated electric fields in front of the display is achieved. However, for displays having separate EHT generation and horizontal deflection circuits, such a system may not achieve cancellation of the field since although the two circuits are usually, but not always, synchronised, they may be distanced from each other in phase.
Prior art methods of using cancellation fields to reduce electric field emissions have used either combined EHT generation and horizontal deflection circuits or separate circuits, but with the circuits in phase as well as synchronised. For these monitors the use of a signal from the horizontal deflection circuit to control the cancellation field provided some reduction in field emissions, but the fact that the primary source of radiated electric fields from the front of the display was the modulation of the internal CRT final anode voltage was not apparent due to the in-phase synchronous nature of the two circuits.
It is advantageous to sense this modulation directly and to provide cancellation based on this modulation rather than based on the horizontal deflection circuit. Even though the prior art method of sensing the field generated by the horizontal deflection in an integrated horizontal deflection and EHT generation circuit will provide some cancellation, improved cancellation can be achieved by sensing the modulation of the CRT anode directly. It is necessary to achieve emission levels of under 1 V/m in order to meet the TCO standard. It is unlikely that such levels can be achieved without eliminating modulations of the CRT final anode voltage.
Unpublished UK Patent Application No. GB9312297.6 describes an open loop active field cancellation system for a CRT display. The system comprises a detection antenna connected via a matching network to the input of an inverting amplifier. The output of the amplifier is connected via a tuning network to a radiating antenna. In operation, the detection antenna detects electric fields radiating from the CRT. The amplifier amplifies and inverts the signal from the detection antenna. The matching network conditions the output from the detection antenna to correct for the amplifier gain and phase characteristics in preparation for application of the inverted signal output from the amplifier to the radiating antenna. A problem with this system is that it requires difficult adjustment during manufacture. Furthermore, in the event of a display fitted with this system requiring a major field service, readjustment may be needed. In addition, the open loop topology of this system limits further reductions in electric field radiation. This is a particularly significant problem because the acceptable electric field emission level may be reduced as research continues. Still furthermore, high precision components are needed to prevent performance degradation with ageing.